What *can't* You Do With Waste Heat?

October 1, 2008

Matthew Wald has a nifty story on the Times' new Green Inc. blog. One notable fact about solar PV systems is that only a fraction of the sun's energy actually gets converted into electricity; the rest is lost as heat waste. But what if there was a way to collect that heat (much of which is collected either by the module itself or in the space underneath) and use it for other purposes—heating buildings or water, say? That's exactly what Vinod Khosla's PVT Solar is trying to do:

The company is currently testing electronic controllers that play traffic cop for the collected heat, pumping it automatically, using a small fan, to the basement hot-water heater, for example, or to individual rooms, or even to the swimming pool, as needs arise. If the heat is not needed in the building, the fan vents it to the outside.

Because solar panels perform better at cooler temperatures, removing heat from around the panels also has the effect of increasing their production on hot days — adding to the overall efficiency gains for the system.

Best part: The controllers are fairly cheap to add, and Khosla claims it can increase the efficiency of the system by over 50 percent. Grist's Dave Roberts comments that "the pursuit of usable waste heat is one of the great unheralded stories of the green shift."

That's a shrewd point; so how about a little more heralding? This ENN story on waste-heat recovery notes that there are some 47,500 smokestacks in the United States that produce waste heat above 260 degrees C—all of that could, in theory, be converted to roughly 50,000 MW of power, equivalent to half of what we get from all nuclear sources right now.

See also Bill McKibben on this unsexy-but-crucial subject. Sean Casten, an expert on waste-heat recovery, estimates that the United States could theoretically generate the same amount of power it does now while using only half the fossil fuel if we simply recycled the bulk of waste heat from factories and utilities. And, in many cases, the obstacles are merely regulatory, no economic. (If, say, factories are prevented from selling the power they generate through waste heat back to the grid or to the guy down the street, there's less incentive to try it.)